The present application relates to an anode having an anode active material layer containing an anode active material and a battery including the anode.
In recent years, portable electronic devices such as combination cameras (videotape recorder), mobile phones, and notebook personal computers have been widely used, and it is strongly demanded to reduce their size and weight and to achieve their long life. Accordingly, as a power source for the portable electronic devices, a battery, in particular a light-weight secondary batter capable of providing a high energy density has been developed.
In particular, a secondary battery using insertion and extraction of lithium for charge and discharge reaction (so-called lithium ion secondary battery) is extremely prospective, since such a lithium ion secondary battery can provide a higher energy density compared to a lead battery and a nickel cadmium battery.
The lithium ion secondary battery includes an anode having a structure in which an anode active material layer containing an anode active material is provided on an anode current collector. As the anode active material, carbon materials have been widely used. However, in recent years, as the high performance and the multi functions of the portable electronic devices are developed, improving the battery capacity is further demanded. Thus, it has been considered to use silicon instead of the carbon materials. Since the theoretical capacity of silicon (4199 mAh/g) is significantly higher than the theoretical capacity of graphite (372 mAh/g), it is prospected that the battery capacity is thereby highly improved.
In the lithium ion secondary battery using silicon as the anode active material, in order to improve the cycle characteristics as important characteristics of the secondary battery, several techniques have been already proposed. Specifically, a technique in which an antioxidant film made of an oxide coat or a polymer coat is provided on the surface of anode active material particles (for example, refer to Japanese Unexamined Patent Application Publication No. 2004-185810), a technique in which ceramics not reacting with lithium is adhered to an anode active material (for example, refer to Japanese Unexamined Patent Application Publication No. 2000-036323), a technique in which a metal oxide is added to or mixed with an anode active material (for example, refer to Japanese Unexamined Patent Application Publication No. 2000-173585) and the like are known. In addition, as a related art, techniques in which particles of a lithium-containing complex nitride or particles of a lithium-containing complex oxide are coated with conductive micro-powder (for example, refer to Japanese Unexamined Patent Application Publication No. 2001-325950) is also known.
The recent portable electronic devices increasingly tend to become small, and the high performance and the multifunctions tend to be increasingly developed. Thus, there is a tendency that the discharge capacity is easily lowered by frequently repeating charge and discharge of the secondary battery. Therefore, it is aspired that the cycle characteristics of the secondary battery could be further improved.
In the existing secondary batteries, however, there is room for improvement of the cycle characteristics. Specifically, when silicon is used as an anode active material in order to improve the battery capacity in the lithium ion secondary battery, a lithium oxide is formed on the surface of the anode due to repetition of charge and discharge. In this case, the lithium oxide is deposited as an irreversible coat. In the result, the absolute number of lithium (lithium ion) contributing to charge and discharge reaction is decreased, and thereby the discharge capacity is lowered.